A security facility of this type in the form of a security thread is known, for example, from WO 95/26884. In this known security thread, which comprises a plastic thread as a support with a covering metal layer, breaks in the metal layer are disposed at right angles to the longitudinal direction of the A thread, so that the conducting metal parts thus formed form areas which are electrically insulated from one another. These metal parts, together with the breaks, form a type of bar code, which can be read with detectors specifically developed for that purpose. Furthermore, this security facility is also machine-readable due to the conducting characteristics of the metal areas.
A similar type of security thread is also already known from GS-A-1353244. In this known security facility, the metal covering layer, which is present on one or both sides of a plastic thread, is similarly broken in a regular manner. If a two-sided metal layer is provided, the position of the breaks can be selected in such a way that a pattern of partially overlapping metal areas is formed. A pattern of this type can be detected in a specific manner.
As well as the aforementioned machine-readable functions, which can be regarded as hidden features, the metallized plastic thread also functions as a public feature. Security threads of this type in fact reveal an optical effect, known in the art as an “optically variable effect”. This effect is based on the fact that a metallized thread, when incorporated into a paper mass, reveals a reflection, which differs only slightly from the reflection of the paper mass itself. The presence of the thread is therefore barely evident in reflected light. However, in transmitted light, the thread reveals itself as a clearly perceptible dark line. This effect is difficult for forgers to imitate using existing copying techniques.
The aforementioned machine-detectable characteristics are based on the normal conduction characteristics of the conducting parts of the thread. However, this conducting behaviour is very simple to imitate by placing a conducting material in the correct position, for which many materials come into consideration, such as, for example, metal-based printing inks and pastes. Even the simplest imitation of a completely hidden metallized plastic security thread, namely a (faint) black-lead strip, shows conduction, since graphite is a good conductor. Similarly, the window-design of a metallized security thread, such as, inter alia, that known from GB-A-1 552 853, EP-A-0 059 056 and DE-A-19 70 604.9, can be imitated, for example by the so-called “stamping” of a metal foil on a banknote. These imitations may reveal electrically conducting behaviour which corresponds to that of the metal-containing security thread, depending on the measurement method which is employed. In practice, therefore, conduction, as a machine-readable characteristic of the security thread, offers only a simple security feature.
Furthermore, it is known that measurement of conduction over longer distances causes problems in a thread with a metal layer on one side only, as a result of the presence of breaks, cracks and the like in the metal. Interruptions of this type may arise as a result of the production method, for example the incorporation of the thread in, for example, a paper substrate, and as a result of daily use. The risk of the occurrence of breaks is even greater in a security thread according to EP-A-0 319 157, in which, in a continuous metal layer, symbols, characters and the like are provided in the form of (metal-free) indentations, which are surrounded by relatively narrow metal parts. These narrow metal parts are particularly prone to breaking.
Furthermore, security threads in which conducting plastics are used are also known. Examples of these are described in EP-A-0 330 733 nd EP-A-0 753 623.